High power diesel engines are commonly equipped with exhaust-driven turbochargers to increase the engine output power by boosting the intake air pressure, and hence the density of the air/fuel mixture in the engine cylinders. Turbocharging can also be used to reduce soot emissions when the engine is operated at higher-than-stoichiometric air/fuel ratios, albeit at the expense of thermodynamic efficiency. Unfortunately, turbocharging also tends to increase the formation of oxides of nitrogen (NOx) due to the increased exhaust gas temperature in the exhaust manifold, and is relatively ineffective at low engine speeds in any event. Accordingly, what is needed is a way of reducing exhaust emissions in a turbocharged diesel engine without sacrificing engine operating efficiency, while at the same time improving turbocharger performance at low engine speeds to make the engine suitable for high torque, low speed operation.